Some years ago, in an investigation on the affinity of organic bases, I was led to study the amount of decomposition suffered by various salts when dissolved in water, for a knowledge of this amount gives us a method of comparing the relative strengths of weak bases. When these bases are made to unite with a given acid, the extent to which the resulting salts are decomposed, in equivalent aqueous solutions, into acid and base, depends directly on the strength of the various bases. The same process of decomposition by water, or hydrolysis, can also be made use of to determine the strengths of weak acids, and experiments in this direction have been executed by Lellmann and his pupils, and by Shields. As the amount of free acid or free base in a hydrolysed solution cannot be determined directly by neutralisation, on account of the progressive disturbance of the chemical equilibrium involved in the process, it must be determined by taking advantage of some property of the acid or base which is susceptible of accurate measurement, and does not destroy the chemical equilibrium, or else does so in a manner amenable to calculation.